Fuel bundles for boiling water nuclear reactors can be generally summarized as to their construction. They include a lower tie plate, an upper tie plate and a plurality of vertically upstanding fuel rods there between. Insofar as is relevant to this invention, a fuel bundle channel surrounds the lower tie plate, extends around the fuel rods to the upper tie plate, and forms a confined fluid flow path unique to each fuel bundle. This flow path between the tie plates is essentially isolated from both other fuel bundles and the surrounding core by-pass region about the fuel bundles.
The lower tie plate supports the fuel rods and permits the entry of water for both the generation of steam and moderation of fast moving electrons from the nuclear reaction in the fuel rods to slower moving electrons to continue the nuclear reaction. The upper tie plate allows the escape of water and generated steam. It is from the escaped steam that power is ultimately generated, this generation occurring typically in a turbine that is remote from the reactor.
The fuel rods within the fuel bundle are long and slender--typically being in the range of 160 inches in length. These fuel rods are all placed within a square matrix 51/4 inches on a side (dimension is approximate). The modern tendency to enable both improved nuclear performance and steam generation efficiency has been to place increasing numbers of discrete fuel rods into the same 51/4 inch square section. Currently densities of fuel rods are known that include 8 by 8, 9 by 9, and 10 by 10, matrices of fuel rods. With the dense fuel bundle arrays, the individual fuel rods become smaller of diameter and the fuel rods become more flexible between the upper and lower tie plate.
Water and steam fluid flow through the fuel bundles causes the long slender fuel rods to become subject to vibration. Without restraint, the fuel bundles could come into abrading contact and lose their required sealing of their contained nuclear fuel. Further, when the fuel rods move out of alignment, they lose designed nuclear efficiency which is realized when the rods maintain precise designed side-by-side spacing. Accordingly, it has long been the accepted practice of the nuclear industry to incorporate so-called fuel rod spacers at varied elevations within the fuel bundles.
The prior art construction of fuel bundle spacers is relatively easy to understand. A spacer intimately surrounds each and every fuel rod at the particular elevation of the spacer. Thus, each spacer at its own elevation causes each fuel rod to be braced into its designed placement relative to the remaining fuel rods. Various schemes are utilized. These schemes include so-called cell spacers in which individual cells surround each fuel rod at the elevation of the spacer to maintain the surrounded fuel rod in its designed orientation. The cells have been round, or as disclosed in the preferred embodiment here, octagonal.
These fuel rod spacers are the sites of pressure loss within the fuel bundle. In order to understand this effect, it is first necessary to understand that water is pumped through the fuel bundles. Thereafter, the problem of the more dense fuel bundle arrays in causing pressure drop at the location of the spacers can be understood.
In fuel bundles, water moderator is generally pumped into the fuel bundles entering through the lower tie plate and exiting upward through the upper tie plate. As water moves through the fuel bundle, the pressure drop of the water in its passage through the fuel bundle becomes important. In so far as is relevant here, pressure drop has an impact on not only the overall power output of the reactor, but additionally impacts the stability of the reactor at certain power and flow rates. By way of specific example, excessive pressure drop in the upper two phase (steam/water) region of the fuel bundle is avoided to prevent certain thermal-hydraulic and nuclear-thermal-hydraulic instabilities that arise at certain moderator flow and power rates of the reactor. In short, there is a continuing effort on the part of nuclear designers to maintain low pressure drop within the fuel bundles of the reactor.
Having explained in general terms the problems of pressure drop within the upward pumped moderator flow within each fuel bundle, the aggravation of pressure drop by new and dense arrays of fuel rods can now be understood.
In the relatively dense and new fuel arrays, spacers constitute areas of flow constriction within the fuel bundle. This effect can be understood by considering the effects of surface friction on the passing moderator as well as the restriction of the available flow area that a spacer represents.
When a fuel bundle has additional fuel rods introduced within the matrix of the array, the surface area of the fuel rods over which flow must occur increases. This increase in surface area is aggravated at each spacer. Remembering that the each fuel rod is discretely surrounded by the material of the spacer at the elevation of the spacer, two additional surfaces are added at the elevation of the spacer to produce fluid friction.
In addition to the friction that the passing fluid encounters from the surface of the fuel rod, the surfaces of the spacer add their own friction. Specifically, the surface of the spacer exposed towards the fuel rod adds flow friction. Likewise, the surface of the spacer exposed away from the fuel rod adds flow friction. These respective surfaces will contribute to flow friction and thus pressure drop approximately proportional to the length of the spacer cells involved.
Further, the total flow area around the fuel rods will be lessened by the dimension of the spacer. This dimension of the spacer adds flow friction roughly proportional to the square of the spacers cross sectional projected area. Due to the cell placement, this spacer construction internally will have one-half the projected area of the prior art type cell spacer.
Moreover, all these flow friction factors come together and act together at the same elevation within the fuel bundle. That elevation is the elevation of the spacer. Thus, it can be understood that the pressure drop produced at the spacers is an important factor in the overall pressure drop of the fuel bundles and the reactor.
Having set forth the pressure drop problem of spacers, the spacer invention of this disclosure can now be described.